This invention relates to a communications system comprising at least two communication units preferably in the form of transceivers, having the same functional characteristics which provide a very high degree of security and privacy. The invention also includes a novel method of secure communications using any apparatus which can accomplish the objectives of the present invention.
In copending application Ser. No. 754,375, filed Aug. 21, 1968 in the name of Walter Ewanus, there is described and claimed a security communications system utilizing at least two communication units having characteristics common to both, such as transceiver components, so that either one can serve as a transmitter or a receiver. In that application, as well as in the present instance, one of the important features is the arrangement of a high speed, bidirectional pseudo-random noise modulator-correlator module between the antenna and the transceiver module. For purposes of simplification, the device heretofore referred to as a modulator-correlator will be called simply a modulator although actually being capable of the dual function as hereinafter outlined. In these systems a pseudo-noise code generator supplies modulation signals to the modulator module when the communication unit is operating in the modulation transmitting mode. The carrier from the transmitter portion of the transceiver is phase coded in the modulator module to produce a very widespread frequency spectrum. A similar modulator module, operating in the correlation, (or receiving) mode between the antenna and the receiver at the receiving unit demodulates the received signal by correlation with the transmitted code. The modulator module of both units will spread the spectrum of any electromagnetic wave signal being received at either of the terminals which is not properly coded so that such signals will be rejected. The modulator module at the transmitting unit, operating in the modulation mode, will spread the spectrum of the propagated carrier wave so that there will be practically no signal power within the aperture of a conventional receiver. This gives antijamming characteristics with a very high degree of rejection of uncorrelated signals. Although the communication units are illustrated by transceiver units, it is obvious that separate transmitter and receiver units may be substituted for the transceiver components.
In the previously mentioned patent application, security and privacy are accomplished to a certain degree by superimposing the intelligence signals directly on the carrier by any type of modulation in which the amplitude of the carrier remains constant, such as in phase, or angle, modulation. However, if AM modulation is used, a conventional AM receiver, which can overcome the processing gain by virtue of its close proximity to the transmitter, can recover the modulating intelligence signals by detecting the amplitude of the pseudo-noise modulated spectrum.
In another copending patent application Ser. No. 54,259 filed July 13, 1970 and assigned to the same assignee as the present application there is described and claimed an improvement over the system described in the previously mentioned patent application in that the invention in Ser. No. 54,259 provides a greater degree of privacy and security by a novel method and means for modulation to provide a constant amplitude spread spectrum carrier from which no intelligence can be recovered without the proper pseudo-random code. The present invention has all the basic advantages of the components in the previously mentioned patent applications in that conventional AM receivers and transmitters can be used and in that the system is such that only the transmitting antenna need have very wide band characteristics to handle the signals. On the other hand, whereas in the prior systems the intelligence can be extracted without the proper code under certain conditions, when the receiver is near enough to the transmitter because there is an AM component on the side bands, in the present invention the side bands do not have any AM intelligence signal components and of course the carrier is suppressed by the phase reversal modulation. In the previously mentioned systems there is an AM signal component on the sidebands and a conventional AM receiver close enough to the transmitting antenna could extract the signal intelligence from the side bands, whereas in the present invention there is no AM signal component on the side bands. Since, in the present invention the signal intelligence is impressed on the carrier in the form of phase modulation, there is no AM signal component on the carrier but by means of the novel modulator/demodulator units of the present invention a conventional AM receiver, when used as a part of the present invention, can extract the signal intelligence from the carrier which signal appears to a conventional AM receiver, not operating in the present system, to be suppressed.
This is one of the salient features of the present invention in that special receiver components are not required to accomplish the advantages of the privacy and security provided by the present system.
The present invention provides a new modulation technique called correlation slope modulation (CSM) which provides a greater degree of privacy over amplitude modulated spread spectrum communication systems, such as those described in the aforementioned patent applications. The unique principle of the present invention is that the relative position in time between the pseudo-noise sequences of the transmitter and receiver is varied in accordance with the amplitude of the intelligence signal source. The resulting transmission is then constant amplitude carrier which is converted to the conventional AM only through correlation at the receiver terminal.